texlive: deprecate texlive.combine and other improvements for 24.11

[xworld21]

Successor of #250243. Now that texlive.withPackages has been tested for a while, I'd like to make a further round of improvements to texlive, and deprecate texlive.combine.

The plan:

• texlive.withPackages improvements
  • [ ] replace buildEnv with mkDerivation (finalAttrs: { ... }): adding docs becomes texliveSmall.overrideAttrs { withDocs = true; }, other customisations become possible via preConfigure etc. hooks (#312945)
  • [ ] deprecate texlive.combine, temporary texliveSmall.__overrideTeXConfig
  • [x] build formats in their own derivations (#291396)
• buildTeXLivePackage improvements
  • [ ] build TeX Live packages with mkDerivation (finalAttrs: { ... }) too, so that overriding can be done via .overrideAttrs (requires fine tuning tlpdb.nix before stabilising)
  • [x] export top level texlivePackages so that all containers are built by Hydra, including docs, and packages appear in search (#289854)
  • [ ] add tlcontrib (#300695)
  • [ ] ...add setup hooks, so that packages can be added as build inputs without the overhead of withPackages (sketched in #251434, but takes finessing around using package sets consistently)

Then right after 24.05, I'll start working on a better way of building the binaries in order to make overrides easy (mostly, make switching between TL 2023 and 2024 easy).

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[xworld21]

So we won't deprecate texlive.combine in 24.05 after all, I started the work too late. But I have the pieces ready to go ahead right after the branch off so we should be good for 24.11.

I might also have some acceptable ideas for merging binaries & packages in a single (overrideable!) scope. That is basically a complete rewrite of the build system, though. I'll put up a separate repo once it looks plausible.